Development and function of the ocular lens depends on controlled proliferation of epithelial cells, and on migration, elongation, adhesion, and cytoarchitecture of the resulting fiber cells. Neither the identity of the molecular mechanisms that regulate these cellular events, nor the potential role of such mechanisms in cataractogenesis, is fully understood. We hypothesize that cytoskeletal signaling pathways comprised of Rho GTPases (Rac and Rho) and ERM (ezrin-radixin-moesin) proteins, play a crucial role in regulating lens fiber cell elongation, migration, polarity, shape, and packing, via organization of fiber cell adhesive interactions and basal/apical membrane complexes. Supporting evidence for this hypothesis derives from recent studies in which targeting Rho GTPase (Rho, Rac and Cdc42) function illuminated a definite role for Rho GTPases in lens development and structural integrity. Further, Rho GTPase-regulated phosphorylation of ERM proteins, which have been identified as major components of the lens fiber cell cytoskeleton, and organization of the cortical actin cytoskeleton and membrane complexes, were noticeably impaired in such lenses. Moreover, in preliminary experiments, three different Rac1 lens conditional knockout (cKO) mouse models developed with Cre recombinase-mediated deletion of the Rac1 gene at different developmental stages and in different regions of the lens, were found to exhibit defective fiber cell migration, sutural defects, and cell-cell junctions. These different observations argue a pivotal role for Rac GTPase, and ERM proteins in regulating fiber cell migration and elongation, and in remodeling of fiber cell cortical adhaerens and basal membrane complex, during lens growth and development. To understand the regulatory role of Rac and ERM proteins in lens cytoarchitecture and function, two specific aims will be pursued in this application: 1. The role of Rac1 GTPase in lens fiber cell migration, basal membrane complex organization, cell-cell adhesion, and in fiber cell elongation will be investigated using lenses derived from the Rac1 cKO mouse models;2. Regulation of ERM protein phosphorylation and ERM protein-dependent remodeling of the fiber cell cortical adhaerens, and fiber cell packing, will be evaluated in Rac1 cKO and Rho GTPase targeted mouse lenses. The completion of these studies should unravel the broad significance of cytoskeletal signaling and the specific role of Rac1 GTPase and ERM proteins in lens fiber cell migration, cytoarchitecture, elongation, and adhesion in normal and cataractous lenses.